In a communication system where switching between the receive and transmit modes is needed, it is very common to encounter frequency pull problems. This problem is more prevalent in Time Division Duplex (TDD) systems where switching between receive and transmit is in the order of milliseconds.
Frequency pull is a phenomenon associated with the Voltage Controlled Oscillator (VCO) where due to impedance variations presented at the output of the oscillator, the oscillation frequency of the VCO will momentarily change frequency, thus potentially adversely affecting the communication link.
Referring to FIG. 1 there is shown a conventional radio circuit 10 in which the frequency pull problem is illustrated. The radio 10 is a time-division multiplexed radio. The radio is in the receive mode for one millisecond and in the transmit mode for one millisecond. In the receive mode, the radio circuit 10 receives radio frequency (RF) signals with an antenna, which is coupled to the receiver and transmitter portions of the radio 10 through an antenna switch 12. A first intermediate frequency (IF) section converts the received RF signals to first IF signals. A receiver down mixer 16 multiplies the first IF signals with a second injection signal provided at a second local oscillator (VCO) 24 to produce second IF signals. A second IF section 18 demodulates the second IF signals and a receiver audio section processes the demodulated signals and applies the resulting signals to a speaker 22.
In the transmit mode, controller 28 turns on the mixer 32 and the power amplifier 30, and a microphone 38 provides audio signals to a transmitter section 36. The transmitter up-mixer 32 multiplies the output of the transmitter section 36 with an injection signal (provided by the VCO 24) to provide an RF signal. A power amplifier 30 amplifies the output of the transmitter up-mixer 32 for transmission by the antenna. Since during the receive mode the transmitter up-mixer 32 is off, it presents a relatively high impedance to the VCO 24. When the radio 10 returns to the transmit mode, the controller 28 turns the transmitter up-mixer 32 back on and it exhibits an impedance of approximately 50 Ohms (i.e., a relatively low impedance). This change in impedance creates a frequency pull problem for the VCO 24. For example, at time "a" (the instant of switching to the receive mode), the VCO 24 frequency changes from its nominal frequency, and takes about a substantial period of the receive or transmit cycle to return to its original frequency. Similarly, the frequency of VCO 34 changes, at time "b" (as shown in the bottom waveform), when the radio switches back to the transmit mode. The impedance variation that causes this frequency change is attributed to the turning on and off of the mixer 32 and the power amplifier 30.
The most common way of solving the problem is by designing buffer stages (one or more), such as buffer 26, in between the oscillator 24 and the switched device(s) (mixer 32 and amplifier 30, in this case). The buffering is used to provide isolation and the changes in impedance are drastically reduced at the output of the oscillator. The buffer solution, however, presents the following disadvantages:
Higher current is needed during standby operation of the radio (i.e., during receive);
Depending on the isolation needed, several stages of buffers might be needed;
The parts count is higher;
Circuit board surface area needed is higher.
Thus, a need exists for a solution to the frequency pull problem that avoids the above disadvantages.